Hydrous nano-silicate melt inclusions supports amphibole growth, Persani Mountains Volcanic Field (Transylvania)

Mantle metasomatism plays an important role in the element transport within the Earth’s mantle causing significant change in rheology and geochemistry of the infiltrated region. During volatile-rich fluid-mediated mantle metasomatism, new volatile-rich phases form at the micro- and nanoscale due to fluid-solid interaction. These micro- and nanoscale processes go hand-in-hand but evidence regarding the similarity or difference is scarce. We studied amphibole lamellae and here the connection should be mentioned nano-silicate melt inclusions on clinopyroxene from an amphibole-bearing mantle xenolith from the Persani Mountains Volcanic Field, southeastern Transylvania (Romania) with transmission electron microscopy (TEM).

Based on petrography, post-entrapment interaction occurred between the host clinopyroxene and the trapped CO₂-rich fluid in the inclusion, which all was essential to form the amphibole lamellae. According to our observation at the nanoscale, fluid escaped from the fluid inclusion followed by amphibole formation along the clinopyroxene-amphibole interface. The escaped fluid formed nano-silicate melt inclusions (NSMIs) that was studied by TEM. These NSMI consist of ~ 80 v% silicate glass and ~ 20 v% bubble. The composition of the silicate glass is as follows: high SiO₂ (>60 wt.%) and Al₂O₃ (>20 wt.%), whereas low CaO, FeO and MgO (sum <8 wt.%). We calculated the original bulk composition of the nano-silicate melt inclusions with Monte Carlo simulation using hydrated fluid complexes. The results show that the nano-silicate melt inclusions originally had a low SiO₂ (~43.5 wt.%) and high Al₂O₃ (~15.5 wt.%), Na₂O (~12.0 wt.%) and H₂O (~30.5 wt.%) content.

The composition of the studied nano-silicate melt inclusion suggests that significant compositional change occurs during nano-scale fluid formation from the parent (micron-scale) CO₂-rich fluid. Therefore, our results suggest that mineral interfaces play a significant role in hydrous mineral precipitation and growth. Furthermore, we propose that nanoscale processes might play significant role in fluid-poor regions and during and after fluid-mediated metasomatic events within the lithospheric mantle (e.g., H₂O, Na, Al consumption during amphibole formation). In addition, the nano-scale fluid migration that we observed here can be one of the mechanisms that enhances deep lithosphere-originated fluid degassing that occurs at many sites on planet Earth. Eventually, our results also provide information about the interactions of H₂O globally in the lithospheric mantle where hydrous minerals are stable and along the lithosphere-asthenosphere boundary in younger oceanic and continental plates.